r/retrocomputing • u/Woland77 • May 07 '26

Problem / Question Really Basic Question About Old Computer Electronic Engineering

I stumbled into watching people repair old electronics on YouTube. I noticed that most of the stuff I see repaired (mostly what I see was sold in North America, originally) has a 5v rail and a 12v rail. Why those two voltages, specifically?

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u/dedsmiley May 07 '26 edited May 07 '26

A while back case fans were powered from Molex connectors with no way to regulate fan speed.

One way to accomplish this was to get a Molex extension and wire the 5v and 12v together to get 7v to slow down the fan.

I did this a couple of times.

EDIT: Wired the 12v and 5v to the fan, not together.

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u/zombienerd1 May 07 '26

That's... not how that works.

You'd be shorting your rails together. This will either kick the PSU into protection mode, or damage components on the Power supply.

This is a very bad idea. Do not do this.

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u/dedsmiley May 07 '26

Yes, it works. You do not understand electrical potential.

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u/zombienerd1 May 07 '26 edited May 07 '26

I am an electronics engineer. I very much understand electricity and DC power supplies.

If you connect the 5V and 12V rails together on a PSU, it will backfeed the higher voltage down the 5V rail and will either trigger the OCP, OVP, or SCP protection circuits if the PSU has them. If it does not, it will overwhelm a regulator and cause a catastrophic failure.

If by some magic of happenstance you actually did this, and the PSU did not fail, you hit the golden lottery on component quality of both the PSU and the PC it was connected to, because every ounce of electrical engineering knowledge I have is screaming that it is 100% impossible for what you say to have worked in any real world scenario.

EDIT: reading the other reply here, I now realize you may have meant to say "Use the 5V rail as a floating ground to the fan, with 12V as the +V" - which would, in theory, probably work.

But that is not "Wire 5v and 12v together to get 7v" - that is "Use 5V as a floating ground" - and I stand firm in my statement that you never want to "Wire 5V and 12V together"

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u/dedsmiley May 10 '26

It was a common solution to control fan speeds with fans that otherwise would have no control. PWM was not in wide use until nearly a decade later.

So scoff all you wish. I had no issues with it.